mutex

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// <mutex> -*- C++ -*-
 
// Copyright (C) 2003-2021 Free Software Foundation, Inc.
//
// This file is part of the GNU ISO C++ Library.  This library is free
// software; you can redistribute it and/or modify it under the
// terms of the GNU General Public License as published by the
// Free Software Foundation; either version 3, or (at your option)
// any later version.
 
// This library is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
// GNU General Public License for more details.
 
// Under Section 7 of GPL version 3, you are granted additional
// permissions described in the GCC Runtime Library Exception, version
// 3.1, as published by the Free Software Foundation.
 
// You should have received a copy of the GNU General Public License and
// a copy of the GCC Runtime Library Exception along with this program;
// see the files COPYING3 and COPYING.RUNTIME respectively.  If not, see
// <http://www.gnu.org/licenses/>.
 
/** @file include/mutex
 *  This is a Standard C++ Library header.
 */
 
#ifndef _GLIBCXX_MUTEX
#define _GLIBCXX_MUTEX 1
 
#pragma GCC system_header
 
#if __cplusplus < 201103L
# include <bits/c++0x_warning.h>
#else
 
#include <tuple>
#include <chrono>
#include <exception>
#include <type_traits>
#include <system_error>
#include <bits/std_mutex.h>
#include <bits/unique_lock.h>
#if ! _GTHREAD_USE_MUTEX_TIMEDLOCK
# include <condition_variable>
# include <thread>
#endif
#include <ext/atomicity.h>     // __gnu_cxx::__is_single_threaded
 
#if defined _GLIBCXX_HAS_GTHREADS && ! defined _GLIBCXX_HAVE_TLS
# include <bits/std_function.h>  // std::function
#endif
 
namespace std _GLIBCXX_VISIBILITY(default)
{
_GLIBCXX_BEGIN_NAMESPACE_VERSION
 
  /**
   * @addtogroup mutexes
   * @{
   */
 
#ifdef _GLIBCXX_HAS_GTHREADS
 
  // Common base class for std::recursive_mutex and std::recursive_timed_mutex
  class __recursive_mutex_base
  {
  protected:
    typedef __gthread_recursive_mutex_t         __native_type;
 
    __recursive_mutex_base(const __recursive_mutex_base&) = delete;
    __recursive_mutex_base& operator=(const __recursive_mutex_base&) = delete;
 
#ifdef __GTHREAD_RECURSIVE_MUTEX_INIT
    __native_type  _M_mutex = __GTHREAD_RECURSIVE_MUTEX_INIT;
 
    __recursive_mutex_base() = default;
#else
    __native_type  _M_mutex;
 
    __recursive_mutex_base()
    {
      // XXX EAGAIN, ENOMEM, EPERM, EBUSY(may), EINVAL(may)
      __GTHREAD_RECURSIVE_MUTEX_INIT_FUNCTION(&_M_mutex);
    }
 
    ~__recursive_mutex_base()
    { __gthread_recursive_mutex_destroy(&_M_mutex); }
#endif
  };
 
  /// The standard recursive mutex type.
  class recursive_mutex : private __recursive_mutex_base
  {
  public:
    typedef __native_type*                      native_handle_type;
 
    recursive_mutex() = default;
    ~recursive_mutex() = default;
 
    recursive_mutex(const recursive_mutex&) = delete;
    recursive_mutex& operator=(const recursive_mutex&) = delete;
 
    void
    lock()
    {
      int __e = __gthread_recursive_mutex_lock(&_M_mutex);
 
      // EINVAL, EAGAIN, EBUSY, EINVAL, EDEADLK(may)
      if (__e)
        __throw_system_error(__e);
    }
 
    bool
    try_lock() noexcept
    {
      // XXX EINVAL, EAGAIN, EBUSY
      return !__gthread_recursive_mutex_trylock(&_M_mutex);
    }
 
    void
    unlock()
    {
      // XXX EINVAL, EAGAIN, EBUSY
      __gthread_recursive_mutex_unlock(&_M_mutex);
    }
 
    native_handle_type
    native_handle() noexcept
    { return &_M_mutex; }
  };
 
#if _GTHREAD_USE_MUTEX_TIMEDLOCK
  template<typename _Derived>
    class __timed_mutex_impl
    {
    protected:
      template<typename _Rep, typename _Period>
        bool
        _M_try_lock_for(const chrono::duration<_Rep, _Period>& __rtime)
        {
#if _GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK
          using __clock = chrono::steady_clock;
#else
          using __clock = chrono::system_clock;
#endif
 
          auto __rt = chrono::duration_cast<__clock::duration>(__rtime);
          if (ratio_greater<__clock::period, _Period>())
            ++__rt;
          return _M_try_lock_until(__clock::now() + __rt);
        }
 
      template<typename _Duration>
        bool
        _M_try_lock_until(const chrono::time_point<chrono::system_clock,
                                                   _Duration>& __atime)
        {
          auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
          auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
 
          __gthread_time_t __ts = {
            static_cast<std::time_t>(__s.time_since_epoch().count()),
            static_cast<long>(__ns.count())
          };
 
          return static_cast<_Derived*>(this)->_M_timedlock(__ts);
        }
 
#ifdef _GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK
      template<typename _Duration>
        bool
        _M_try_lock_until(const chrono::time_point<chrono::steady_clock,
                                                   _Duration>& __atime)
        {
          auto __s = chrono::time_point_cast<chrono::seconds>(__atime);
          auto __ns = chrono::duration_cast<chrono::nanoseconds>(__atime - __s);
 
          __gthread_time_t __ts = {
            static_cast<std::time_t>(__s.time_since_epoch().count()),
            static_cast<long>(__ns.count())
          };
 
          return static_cast<_Derived*>(this)->_M_clocklock(CLOCK_MONOTONIC,
                                                            __ts);
        }
#endif
 
      template<typename _Clock, typename _Duration>
        bool
        _M_try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime)
        {
#if __cplusplus > 201703L
          static_assert(chrono::is_clock_v<_Clock>);
#endif
          // The user-supplied clock may not tick at the same rate as
          // steady_clock, so we must loop in order to guarantee that
          // the timeout has expired before returning false.
          auto __now = _Clock::now();
          do {
            auto __rtime = __atime - __now;
            if (_M_try_lock_for(__rtime))
              return true;
            __now = _Clock::now();
          } while (__atime > __now);
          return false;
        }
    };
 
  /// The standard timed mutex type.
  class timed_mutex
  : private __mutex_base, public __timed_mutex_impl<timed_mutex>
  {
  public:
    typedef __native_type*                      native_handle_type;
 
    timed_mutex() = default;
    ~timed_mutex() = default;
 
    timed_mutex(const timed_mutex&) = delete;
    timed_mutex& operator=(const timed_mutex&) = delete;
 
    void
    lock()
    {
      int __e = __gthread_mutex_lock(&_M_mutex);
 
      // EINVAL, EAGAIN, EBUSY, EINVAL, EDEADLK(may)
      if (__e)
        __throw_system_error(__e);
    }
 
    bool
    try_lock() noexcept
    {
      // XXX EINVAL, EAGAIN, EBUSY
      return !__gthread_mutex_trylock(&_M_mutex);
    }
 
    template <class _Rep, class _Period>
      bool
      try_lock_for(const chrono::duration<_Rep, _Period>& __rtime)
      { return _M_try_lock_for(__rtime); }
 
    template <class _Clock, class _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime)
      { return _M_try_lock_until(__atime); }
 
    void
    unlock()
    {
      // XXX EINVAL, EAGAIN, EBUSY
      __gthread_mutex_unlock(&_M_mutex);
    }
 
    native_handle_type
    native_handle() noexcept
    { return &_M_mutex; }
 
    private:
      friend class __timed_mutex_impl<timed_mutex>;
 
      bool
      _M_timedlock(const __gthread_time_t& __ts)
      { return !__gthread_mutex_timedlock(&_M_mutex, &__ts); }
 
#if _GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK
      bool
      _M_clocklock(clockid_t clockid, const __gthread_time_t& __ts)
      { return !pthread_mutex_clocklock(&_M_mutex, clockid, &__ts); }
#endif
  };
 
  /// recursive_timed_mutex
  class recursive_timed_mutex
  : private __recursive_mutex_base,
    public __timed_mutex_impl<recursive_timed_mutex>
  {
  public:
    typedef __native_type*                      native_handle_type;
 
    recursive_timed_mutex() = default;
    ~recursive_timed_mutex() = default;
 
    recursive_timed_mutex(const recursive_timed_mutex&) = delete;
    recursive_timed_mutex& operator=(const recursive_timed_mutex&) = delete;
 
    void
    lock()
    {
      int __e = __gthread_recursive_mutex_lock(&_M_mutex);
 
      // EINVAL, EAGAIN, EBUSY, EINVAL, EDEADLK(may)
      if (__e)
        __throw_system_error(__e);
    }
 
    bool
    try_lock() noexcept
    {
      // XXX EINVAL, EAGAIN, EBUSY
      return !__gthread_recursive_mutex_trylock(&_M_mutex);
    }
 
    template <class _Rep, class _Period>
      bool
      try_lock_for(const chrono::duration<_Rep, _Period>& __rtime)
      { return _M_try_lock_for(__rtime); }
 
    template <class _Clock, class _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime)
      { return _M_try_lock_until(__atime); }
 
    void
    unlock()
    {
      // XXX EINVAL, EAGAIN, EBUSY
      __gthread_recursive_mutex_unlock(&_M_mutex);
    }
 
    native_handle_type
    native_handle() noexcept
    { return &_M_mutex; }
 
    private:
      friend class __timed_mutex_impl<recursive_timed_mutex>;
 
      bool
      _M_timedlock(const __gthread_time_t& __ts)
      { return !__gthread_recursive_mutex_timedlock(&_M_mutex, &__ts); }
 
#ifdef _GLIBCXX_USE_PTHREAD_MUTEX_CLOCKLOCK
      bool
      _M_clocklock(clockid_t clockid, const __gthread_time_t& __ts)
      { return !pthread_mutex_clocklock(&_M_mutex, clockid, &__ts); }
#endif
  };
 
#else // !_GTHREAD_USE_MUTEX_TIMEDLOCK
 
  /// timed_mutex
  class timed_mutex
  {
    mutex               _M_mut;
    condition_variable  _M_cv;
    bool                _M_locked = false;
 
  public:
 
    timed_mutex() = default;
    ~timed_mutex() { __glibcxx_assert( !_M_locked ); }
 
    timed_mutex(const timed_mutex&) = delete;
    timed_mutex& operator=(const timed_mutex&) = delete;
 
    void
    lock()
    {
      unique_lock<mutex> __lk(_M_mut);
      _M_cv.wait(__lk, [&]{ return !_M_locked; });
      _M_locked = true;
    }
 
    bool
    try_lock()
    {
      lock_guard<mutex> __lk(_M_mut);
      if (_M_locked)
        return false;
      _M_locked = true;
      return true;
    }
 
    template<typename _Rep, typename _Period>
      bool
      try_lock_for(const chrono::duration<_Rep, _Period>& __rtime)
      {
        unique_lock<mutex> __lk(_M_mut);
        if (!_M_cv.wait_for(__lk, __rtime, [&]{ return !_M_locked; }))
          return false;
        _M_locked = true;
        return true;
      }
 
    template<typename _Clock, typename _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime)
      {
        unique_lock<mutex> __lk(_M_mut);
        if (!_M_cv.wait_until(__lk, __atime, [&]{ return !_M_locked; }))
          return false;
        _M_locked = true;
        return true;
      }
 
    void
    unlock()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_locked );
      _M_locked = false;
      _M_cv.notify_one();
    }
  };
 
  /// recursive_timed_mutex
  class recursive_timed_mutex
  {
    mutex               _M_mut;
    condition_variable  _M_cv;
    thread::id          _M_owner;
    unsigned            _M_count = 0;
 
    // Predicate type that tests whether the current thread can lock a mutex.
    struct _Can_lock
    {
      // Returns true if the mutex is unlocked or is locked by _M_caller.
      bool
      operator()() const noexcept
      { return _M_mx->_M_count == 0 || _M_mx->_M_owner == _M_caller; }
 
      const recursive_timed_mutex* _M_mx;
      thread::id _M_caller;
    };
 
  public:
 
    recursive_timed_mutex() = default;
    ~recursive_timed_mutex() { __glibcxx_assert( _M_count == 0 ); }
 
    recursive_timed_mutex(const recursive_timed_mutex&) = delete;
    recursive_timed_mutex& operator=(const recursive_timed_mutex&) = delete;
 
    void
    lock()
    {
      auto __id = this_thread::get_id();
      _Can_lock __can_lock{this, __id};
      unique_lock<mutex> __lk(_M_mut);
      _M_cv.wait(__lk, __can_lock);
      if (_M_count == -1u)
        __throw_system_error(EAGAIN); // [thread.timedmutex.recursive]/3
      _M_owner = __id;
      ++_M_count;
    }
 
    bool
    try_lock()
    {
      auto __id = this_thread::get_id();
      _Can_lock __can_lock{this, __id};
      lock_guard<mutex> __lk(_M_mut);
      if (!__can_lock())
        return false;
      if (_M_count == -1u)
        return false;
      _M_owner = __id;
      ++_M_count;
      return true;
    }
 
    template<typename _Rep, typename _Period>
      bool
      try_lock_for(const chrono::duration<_Rep, _Period>& __rtime)
      {
        auto __id = this_thread::get_id();
        _Can_lock __can_lock{this, __id};
        unique_lock<mutex> __lk(_M_mut);
        if (!_M_cv.wait_for(__lk, __rtime, __can_lock))
          return false;
        if (_M_count == -1u)
          return false;
        _M_owner = __id;
        ++_M_count;
        return true;
      }
 
    template<typename _Clock, typename _Duration>
      bool
      try_lock_until(const chrono::time_point<_Clock, _Duration>& __atime)
      {
        auto __id = this_thread::get_id();
        _Can_lock __can_lock{this, __id};
        unique_lock<mutex> __lk(_M_mut);
        if (!_M_cv.wait_until(__lk, __atime, __can_lock))
          return false;
        if (_M_count == -1u)
          return false;
        _M_owner = __id;
        ++_M_count;
        return true;
      }
 
    void
    unlock()
    {
      lock_guard<mutex> __lk(_M_mut);
      __glibcxx_assert( _M_owner == this_thread::get_id() );
      __glibcxx_assert( _M_count > 0 );
      if (--_M_count == 0)
        {
          _M_owner = {};
          _M_cv.notify_one();
        }
    }
  };
 
#endif
#endif // _GLIBCXX_HAS_GTHREADS
 
  /// @cond undocumented
  template<typename _Lock>
    inline unique_lock<_Lock>
    __try_to_lock(_Lock& __l)
    { return unique_lock<_Lock>{__l, try_to_lock}; }
 
  template<int _Idx, bool _Continue = true>
    struct __try_lock_impl
    {
      template<typename... _Lock>
        static void
        __do_try_lock(tuple<_Lock&...>& __locks, int& __idx)
        {
          __idx = _Idx;
          auto __lock = std::__try_to_lock(std::get<_Idx>(__locks));
          if (__lock.owns_lock())
            {
              constexpr bool __cont = _Idx + 2 < sizeof...(_Lock);
              using __try_locker = __try_lock_impl<_Idx + 1, __cont>;
              __try_locker::__do_try_lock(__locks, __idx);
              if (__idx == -1)
                __lock.release();
            }
        }
    };
 
  template<int _Idx>
    struct __try_lock_impl<_Idx, false>
    {
      template<typename... _Lock>
        static void
        __do_try_lock(tuple<_Lock&...>& __locks, int& __idx)
        {
          __idx = _Idx;
          auto __lock = std::__try_to_lock(std::get<_Idx>(__locks));
          if (__lock.owns_lock())
            {
              __idx = -1;
              __lock.release();
            }
        }
    };
  /// @endcond
 
  /** @brief Generic try_lock.
   *  @param __l1 Meets Lockable requirements (try_lock() may throw).
   *  @param __l2 Meets Lockable requirements (try_lock() may throw).
   *  @param __l3 Meets Lockable requirements (try_lock() may throw).
   *  @return Returns -1 if all try_lock() calls return true. Otherwise returns
   *          a 0-based index corresponding to the argument that returned false.
   *  @post Either all arguments are locked, or none will be.
   *
   *  Sequentially calls try_lock() on each argument.
   */
  template<typename _Lock1, typename _Lock2, typename... _Lock3>
    int
    try_lock(_Lock1& __l1, _Lock2& __l2, _Lock3&... __l3)
    {
      int __idx;
      auto __locks = std::tie(__l1, __l2, __l3...);
      __try_lock_impl<0>::__do_try_lock(__locks, __idx);
      return __idx;
    }
 
  /** @brief Generic lock.
   *  @param __l1 Meets Lockable requirements (try_lock() may throw).
   *  @param __l2 Meets Lockable requirements (try_lock() may throw).
   *  @param __l3 Meets Lockable requirements (try_lock() may throw).
   *  @throw An exception thrown by an argument's lock() or try_lock() member.
   *  @post All arguments are locked.
   *
   *  All arguments are locked via a sequence of calls to lock(), try_lock()
   *  and unlock().  If the call exits via an exception any locks that were
   *  obtained will be released.
   */
  template<typename _L1, typename _L2, typename... _L3>
    void
    lock(_L1& __l1, _L2& __l2, _L3&... __l3)
    {
      while (true)
        {
          using __try_locker = __try_lock_impl<0, sizeof...(_L3) != 0>;
          unique_lock<_L1> __first(__l1);
          int __idx;
          auto __locks = std::tie(__l2, __l3...);
          __try_locker::__do_try_lock(__locks, __idx);
          if (__idx == -1)
            {
              __first.release();
              return;
            }
        }
    }
 
#if __cplusplus >= 201703L
#define __cpp_lib_scoped_lock 201703
  /** @brief A scoped lock type for multiple lockable objects.
   *
   * A scoped_lock controls mutex ownership within a scope, releasing
   * ownership in the destructor.
   */
  template<typename... _MutexTypes>
    class scoped_lock
    {
    public:
      explicit scoped_lock(_MutexTypes&... __m) : _M_devices(std::tie(__m...))
      { std::lock(__m...); }
 
      explicit scoped_lock(adopt_lock_t, _MutexTypes&... __m) noexcept
      : _M_devices(std::tie(__m...))
      { } // calling thread owns mutex
 
      ~scoped_lock()
      { std::apply([](auto&... __m) { (__m.unlock(), ...); }, _M_devices); }
 
      scoped_lock(const scoped_lock&) = delete;
      scoped_lock& operator=(const scoped_lock&) = delete;
 
    private:
      tuple<_MutexTypes&...> _M_devices;
    };
 
  template<>
    class scoped_lock<>
    {
    public:
      explicit scoped_lock() = default;
      explicit scoped_lock(adopt_lock_t) noexcept { }
      ~scoped_lock() = default;
 
      scoped_lock(const scoped_lock&) = delete;
      scoped_lock& operator=(const scoped_lock&) = delete;
    };
 
  template<typename _Mutex>
    class scoped_lock<_Mutex>
    {
    public:
      using mutex_type = _Mutex;
 
      explicit scoped_lock(mutex_type& __m) : _M_device(__m)
      { _M_device.lock(); }
 
      explicit scoped_lock(adopt_lock_t, mutex_type& __m) noexcept
      : _M_device(__m)
      { } // calling thread owns mutex
 
      ~scoped_lock()
      { _M_device.unlock(); }
 
      scoped_lock(const scoped_lock&) = delete;
      scoped_lock& operator=(const scoped_lock&) = delete;
 
    private:
      mutex_type&  _M_device;
    };
#endif // C++17
 
#ifdef _GLIBCXX_HAS_GTHREADS
  /// Flag type used by std::call_once
  struct once_flag
  {
    constexpr once_flag() noexcept = default;
 
    /// Deleted copy constructor
    once_flag(const once_flag&) = delete;
    /// Deleted assignment operator
    once_flag& operator=(const once_flag&) = delete;
 
  private:
    // For gthreads targets a pthread_once_t is used with pthread_once, but
    // for most targets this doesn't work correctly for exceptional executions.
    __gthread_once_t _M_once = __GTHREAD_ONCE_INIT;
 
    struct _Prepare_execution;
 
    template<typename _Callable, typename... _Args>
      friend void
      call_once(once_flag& __once, _Callable&& __f, _Args&&... __args);
  };
 
  /// @cond undocumented
# ifdef _GLIBCXX_HAVE_TLS
  // If TLS is available use thread-local state for the type-erased callable
  // that is being run by std::call_once in the current thread.
  extern __thread void* __once_callable;
  extern __thread void (*__once_call)();
 
  // RAII type to set up state for pthread_once call.
  struct once_flag::_Prepare_execution
  {
    template<typename _Callable>
      explicit
      _Prepare_execution(_Callable& __c)
      {
        // Store address in thread-local pointer:
        __once_callable = std::__addressof(__c);
        // Trampoline function to invoke the closure via thread-local pointer:
        __once_call = [] { (*static_cast<_Callable*>(__once_callable))(); };
      }
 
    ~_Prepare_execution()
    {
      // PR libstdc++/82481
      __once_callable = nullptr;
      __once_call = nullptr;
    }
 
    _Prepare_execution(const _Prepare_execution&) = delete;
    _Prepare_execution& operator=(const _Prepare_execution&) = delete;
  };
 
# else
  // Without TLS use a global std::mutex and store the callable in a
  // global std::function.
  extern function<void()> __once_functor;
 
  extern void
  __set_once_functor_lock_ptr(unique_lock<mutex>*);
 
  extern mutex&
  __get_once_mutex();
 
  // RAII type to set up state for pthread_once call.
  struct once_flag::_Prepare_execution
  {
    template<typename _Callable>
      explicit
      _Prepare_execution(_Callable& __c)
      {
        // Store the callable in the global std::function
        __once_functor = __c;
        __set_once_functor_lock_ptr(&_M_functor_lock);
      }
 
    ~_Prepare_execution()
    {
      if (_M_functor_lock)
        __set_once_functor_lock_ptr(nullptr);
    }
 
  private:
    // XXX This deadlocks if used recursively (PR 97949)
    unique_lock<mutex> _M_functor_lock{__get_once_mutex()};
 
    _Prepare_execution(const _Prepare_execution&) = delete;
    _Prepare_execution& operator=(const _Prepare_execution&) = delete;
  };
# endif
  /// @endcond
 
  // This function is passed to pthread_once by std::call_once.
  // It runs __once_call() or __once_functor().
  extern "C" void __once_proxy(void);
 
  /// Invoke a callable and synchronize with other calls using the same flag
  template<typename _Callable, typename... _Args>
    void
    call_once(once_flag& __once, _Callable&& __f, _Args&&... __args)
    {
      // Closure type that runs the function
      auto __callable = [&] {
          std::__invoke(std::forward<_Callable>(__f),
                        std::forward<_Args>(__args)...);
      };
 
      once_flag::_Prepare_execution __exec(__callable);
 
      // XXX pthread_once does not reset the flag if an exception is thrown.
      if (int __e = __gthread_once(&__once._M_once, &__once_proxy))
        __throw_system_error(__e);
    }
 
#else // _GLIBCXX_HAS_GTHREADS
 
  /// Flag type used by std::call_once
  struct once_flag
  {
    constexpr once_flag() noexcept = default;
 
    /// Deleted copy constructor
    once_flag(const once_flag&) = delete;
    /// Deleted assignment operator
    once_flag& operator=(const once_flag&) = delete;
 
  private:
    // There are two different std::once_flag interfaces, abstracting four
    // different implementations.
    // The single-threaded interface uses the _M_activate() and _M_finish(bool)
    // functions, which start and finish an active execution respectively.
    // See [thread.once.callonce] in C++11 for the definition of
    // active/passive/returning/exceptional executions.
    enum _Bits : int { _Init = 0, _Active = 1, _Done = 2 };
 
    int _M_once = _Bits::_Init;
 
    // Check to see if all executions will be passive now.
    bool
    _M_passive() const noexcept;
 
    // Attempts to begin an active execution.
    bool _M_activate();
 
    // Must be called to complete an active execution.
    // The argument is true if the active execution was a returning execution,
    // false if it was an exceptional execution.
    void _M_finish(bool __returning) noexcept;
 
    // RAII helper to call _M_finish.
    struct _Active_execution
    {
      explicit _Active_execution(once_flag& __flag) : _M_flag(__flag) { }
 
      ~_Active_execution() { _M_flag._M_finish(_M_returning); }
 
      _Active_execution(const _Active_execution&) = delete;
      _Active_execution& operator=(const _Active_execution&) = delete;
 
      once_flag& _M_flag;
      bool _M_returning = false;
    };
 
    template<typename _Callable, typename... _Args>
      friend void
      call_once(once_flag& __once, _Callable&& __f, _Args&&... __args);
  };
 
  // Inline definitions of std::once_flag members for single-threaded targets.
 
  inline bool
  once_flag::_M_passive() const noexcept
  { return _M_once == _Bits::_Done; }
 
  inline bool
  once_flag::_M_activate()
  {
    if (_M_once == _Bits::_Init) [[__likely__]]
      {
        _M_once = _Bits::_Active;
        return true;
      }
    else if (_M_passive()) // Caller should have checked this already.
      return false;
    else
      __throw_system_error(EDEADLK);
  }
 
  inline void
  once_flag::_M_finish(bool __returning) noexcept
  { _M_once = __returning ? _Bits::_Done : _Bits::_Init; }
 
  /// Invoke a callable and synchronize with other calls using the same flag
  template<typename _Callable, typename... _Args>
    inline void
    call_once(once_flag& __once, _Callable&& __f, _Args&&... __args)
    {
      if (__once._M_passive())
        return;
      else if (__once._M_activate())
        {
          once_flag::_Active_execution __exec(__once);
 
          // _GLIBCXX_RESOLVE_LIB_DEFECTS
          // 2442. call_once() shouldn't DECAY_COPY()
          std::__invoke(std::forward<_Callable>(__f),
                        std::forward<_Args>(__args)...);
 
          // __f(__args...) did not throw
          __exec._M_returning = true;
        }
    }
#endif // _GLIBCXX_HAS_GTHREADS
 
  /// @} group mutexes
_GLIBCXX_END_NAMESPACE_VERSION
} // namespace
 
#endif // C++11
 
#endif // _GLIBCXX_MUTEX